Production theory is concerned with finding the most efficient combination of inputs to produce goods and services given constraints. Inputs include labor, capital, and land, which are used to produce outputs in the form of intermediate and final goods. The production function shows the relationship between inputs and maximum possible output in the short and long run. In the short run, at least one input is fixed, so output depends on the variable input. The law of diminishing returns states that adding more of the variable input initially increases, then decreases, marginal product.